Chemical reinforced glass substrate having desirable edge profile and method of manufacturing the same

ABSTRACT

In a method of manufacturing a chemical reinforced glass substrate, consideration is previously made about a relationship between conditions of chemical reinforcement for a glass substrate and deformation caused by the chemical reinforcement at an edge portion of the glass substrate. The glass substrate is chemically reinforced on the basis of the relationship so that an edge profile is shaped into a desirable edge profile during the chemical reinforcement. The resultant chemical reinforced glass substrate is flat and smooth in a wide area and effective to improve a recording density and to avoid head crashes.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a chemical reinforced glass substrateand an information recording medium including the glass substrate andalso to a method of manufacturing the chemical reinforced glasssubstrate and the information medium.

[0002] As one of information recording media, a magnetic disk mounted ona hard disk drive (HDD) is known in the art.

[0003] Recently, it is strongly required to increase storage capacity ofthe magnetic disk. As a result, an extension of a recording region onthe magnetic disk and a high recording density of the recording becomean emergency matter.

[0004] The hard disk drive includes a magnetic head that faces arecording surface of the magnetic disk and flies over the recordingsurface to write/read information to/from the magnetic disk. It isdesirable that a flying height or an interval between the magnetic headand the recording surface is lower and lower because the recordingdensity of the magnetic disk can be increased.

[0005] Two methods are known as a driving method for driving the harddisk drive. One is a CSS (Contact Start and Stop) method and the otheris an LUL (Load/Unload) method. Because the LUL method permits areduction of the interval between the magnetic head and the recordingsurface in comparison with the CSS method, the former enables anincrease of the recording density in comparison with the CSS method.

[0006] On the other hand, the recording surface of the magnetic diskmust be flat and smooth to stabilize a flight of the magnetic head. Thatis, the magnetic disk must have a substrate that has a flat and smoothsurface. As the substrate, attention has been directed to a glasssubstrate for the magnetic disk because its main surface can be madevery flat and very smooth.

[0007] The main surface of the glass substrate is polished by a softpolisher to even or smooth them. However, polishing by the use of thesoft polisher causes a surface down or a surface rise to occur at anouter edge portion and/or an inner edge portion of the glass substrate.In the LUL method, the magnetic head stays at the outside in a radialdirection of the magnetic disk when the magnetic disk is not driven.When the magnetic disk is driven, the magnetic head moves toward thecenter of the magnetic disk to write/read information to/from themagnetic disk and faces the recording surface of the magnetic disk.Accordingly, the surface down and/or the surface rise at the outer edgeportion make the flight of the magnetic head unstable. In the worstcase, the magnetic head clashes with the surface of the magnetic disk.In addition, the down and/or the rise limits the recording area of themagnetic disk. This is because an extent of the recording area dependson a flat area of the main surface of the glass substrate.

[0008] To solve the above-mentioned problems, several proposals havealready been made. For example, a technique for reducing the surfacedown and the surface rise is disclosed in Japanese Unexamined PatentPublication (JP-A) No. H05-89459. The technique provides appropriatepolishing conditions (i.e. polishing pressure and polishing time).Moreover, another technique for stabilizing the flight of the magnetichead is disclosed in Japanese Unexamined Patent Publication (JP-A) No.H05-290365. The technique provides an appropriate radius of curvature atthe outer edge portion of the glass substrate.

[0009] By the way, the glass substrate is often subjected to chemicalreinforcement or treatment used by chemical solution after the polishingprocess to improve mechanical strength and durability. Such a glasssubstrate will be called a chemical reinforced glass substratehereinafter. The chemical reinforcement partially replaces specific ionsincluded in the surface of the glass substrate with other ions largerthan the specific ions in ionic radii.

[0010] According to the inventors' experimental studies, it has beenfound out that the chemical reinforced glass substrate can notaccomplish a stable flight of a magnetic head due to head crashes andthe like and therefore makes it impossible to widen a recording area ofthe magnetic recording medium manufactured from the chemical reinforcedglass substrate.

SUMMARY OF THE INVENTION

[0011] It is therefore an object of this invention to provide a chemicalreinforced glass substrate which is subjected to chemical reinforcementand which has desirable edge shapes or profiles at outer and inneredges.

[0012] It is another object of this invention to provide a chemicalreinforced glass substrate of the type described having a highmechanical strength and a flat and smooth surface.

[0013] It is still another object of this invention to provide achemical reinforced glass substrate which can widen a recording areaformed on its main surface.

[0014] It is further still another object of this invention to provide achemical reinforced glass substrate used for an information recordingmedium which can be properly clamped by a clamp for an informationrecording apparatus.

[0015] It is yet another object of this invention to provide a chemicalreinforced glass substrate used for an information recording mediumwhich enables a writing/reading head to fly stably over its recordingsurface.

[0016] The ski-jump portions at the outer edge portion of the glasssubstrate make the flight of the magnetic head unstable and limit therecording area. In addition, it is unavoidable to enlarge the intervalbetween the magnetic head and the recording surface of the magnetic diskbecause the ski-jump portions at the outer edge portion might bringabout the head clashes with the magnetic disk.

[0017] On the other hand, the ski-jump portions at the inner edgeportion of the glass substrate inclines the magnetic disk against aclamp for clamping the magnetic disk. There is a case where the ski-jumpportions at the inner edge portion distort or destroy the magnetic diskwhen it is clamped by the clamp.

[0018] Herein, description will be made about principles of thisinvention for a better understanding of this invention. Heretofore, achemical reinforced glass substrate is rarely used for manufacturing amagnetic recording medium or has not been investigated aboutcharacteristics of the chemical reinforced glass substrate. According tothe inventors' experimental studies, it has been found out that themagnetic recording medium which includes a chemical reinforced glasssubstrate can not accomplish a stable flight of a magnetic head andoften causes head crashes to occur during read/write operation. As aresult, it is difficult to accomplish a low flight operation of themagnetic head and to expand a recording area of the magnetic recordingmedium.

[0019] Further inventors' research has revealed that the glass substrateis undesirably deformed at inner and outer edge portions of a mainsurface when the glass substrate is subjected to chemical reinforcement.It has been also found out that deformed portions appear in the form ofprojections or recesses at the inner and the outer edge portions. In anyevent, such projections and recesses provide inner and/or outer edgeprofiles and will be referred to as ski-jump portions and roll-offportions, respectively. The ski-jump portions at the outer edge portionof the glass substrate make the flight of the magnetic head unstable andlimit the recording area. In addition, it is unavoidable to enlarge theinterval between the magnetic head and the recording surface of themagnetic disk because the ski-jump portions at the outer edge portionmight bring about the head clashes with the magnetic disk.

[0020] On the other hand, the ski-jump portions at the inner edgeportion of the glass substrate inclines the magnetic disk against aclamp for clamping the magnetic disk. This means that the ski-jumpportions at the inner edge portion distort or destroy the magnetic diskwhen it is clamped by a clamp.

[0021] The ski-jump portions at the outer edge portion of the glasssubstrate make the flight of the magnetic head unstable and limit therecording area. In addition, it is unavoidable to enlarge the intervalbetween the magnetic head and the recording surface of the magnetic diskbecause the ski-jump portions at the outer edge portion might bringabout the head clashes with the magnetic disk.

[0022] According to a first aspect of this invention, a method is foruse in manufacturing a glass substrate for an information recordingmedium, the glass substrate having an edge portion adjacent to an outerand/or an inner peripheral side end. The method comprises the steps ofpreviously finding a relationship between chemical reinforcementconditions and that profile variation at the edge portion of the glasssubstrate which results from chemical reinforcement and performing thechemical reinforcement of the glass substrate on the basis of therelationship to obtain a chemically reinforced glass substrate.

[0023] As mentioned before, consideration is previously made about therelationship between the profile variation caused by the chemicalreinforcement and the chemical reinforcement conditions. The profilevariation may occur, for example, on the edge portion adjacent to theouter peripheral side end and may appear in a thickness direction. Inthis event, the profile variation may be represented by a variablecomponent in the thickness direction. At any rate, the chemicalreinforcement is performed on the basis of the relationship previouslydetected or found and, as a result, the profile variation on the edgeportion adjacent to the outer peripheral side end can be controlled bythe chemical reinforcement condition. This applies to the edge portionadjacent to the inner peripheral side end.

[0024] The chemical reinforcement may be performed, for example, by afirst method of chemically reinforcing a glass substrate by using ionexchange and by a second method of chemically reinforcing the glasssubstrate by using a de-alkali process. Specifically, the first methodrealizes the chemical reinforcement of the glass substrate by exchangingions included in a surface layer of the glass substrate for ions whichare included in a chemical reinforcement solution and which havediameters greater than the ions of the surface layer. With this method,expansion takes place in an in-plane direction of the glass substrateand a profile variation component on the main surface is represented bya positive value. Such a positive value of the profile variationcomponent brings about a surface rise area.

[0025] On the other hand, the second method which uses the de-alkaliprocess shrinks the glass substrate in the in-plane direction and theprofile variation component is represented by a negative value. Thisbrings about a surface down area. In addition, it is confirmed that theprofile variation component of the edge portion adjacent to the innerperipheral side end is smaller than that of the outer peripheral sideend by 10% -20%.

[0026] According to a second aspect of this invention, a method is foruse in manufacturing a glass substrate for an information recordingmedium. The glass substrate has an edge portion adjacent to an outerand/or an inner peripheral side end. The method comprises the steps ofpreviously finding a relationship between chemical reinforcementconditions and that profile variation at the edge portion of the glasssubstrate which results from chemical reinforcement, deciding a profileon the edge portion by predicting the profile variation of the edgeportion to obtain, as a glass substrate prior to chemical reinforcement,a glass substrate prior to chemical reinforcement which has a decidedprofile and which is not subjected to the chemical reinforcement, andperforming the chemical reinforcement of the glass substrate prior tochemical reinforcement to obtain the glass substrate which has a desiredprofile at the edge portion.

[0027] In addition to the merits mentioned in conjunction with the firstaspect, the second aspect of this invention predicts the profilevariation components caused by the chemical reinforcement and uses theglass substrate prior to chemical reinforcement that can cancel them.With this method, it is possible to strictly and precisely control anouter peripheral contour of the glass substrate subjected to thechemical reinforcement. This applies to an inner peripheral contour ofthe glass substrate.

[0028] According to a third aspect of this invention, the chemicalreinforcement performing step is performed on conditions such that theprofile variation on the edge portion becomes small. For example, thechemical reinforcement is carried out so that the profile variationbecomes small on the outer peripheral side end. In this case, it ispossible to suppress, on the outer peripheral side end, the profilevariation which might occur due to the chemical reinforcement, if theglass substrate prior to chemical reinforcement is flat on the outerperipheral side end.

[0029] Alternatively, even when use is made of the glass substrate priorto chemical reinforcement which can cancel the profile variation, asmentioned in the second aspect, a profile variation extremely becomessmall between the glass substrate prior to chemical reinforcement andthe glass substrate. This means that the outer edge profile can readilybe controlled as compared with a large profile variation and stableprocessing can be executed with the profile variation kept small. Theseapply to the inner peripheral side end of the glass substrate.

[0030] According to a fourth aspect of this invention, the chemicalreinforcement performing step is performed under the chemicalreinforcement condition such that a compressive stress layer formed on asurface layer of the glass substrate by the chemical reinforcementreaches to a depth between 3 and 100 μm and has a compressive stress of1-15 kg/mm² and that a tensile stress caused by the chemicalreinforcement within the glass substrate is not larger than 4.5 kg/mm².

[0031] As mentioned before, the compressive stress layer at firstreaches to the depth between 3 and 100 μm. This makes it possible tokeep desirable mechanical strength of the glass substrate and to reducethe profile variation component on the outer peripheral side end whenthe chemical reinforcement is performed.

[0032] When the depth of the compressive stress layer is thinner than 3μm, the mechanical strength becomes undesirably weak in durability andagainst breakage. When the depth of the compressive stress layer exceeds100 μm, the profile variation component becomes large when the chemicalreinforcement is performed. Preferably, the depth of the falls within arange between 40 and 80 μm and more preferably, within a range between50 and 70 μm.

[0033] Second, the compressive stress caused in the surface layer of theglass substrate by the chemical reinforcement is selected between 1 and15 kg/mm² while the tensile stress caused within the glass substrate isnot greater than 4.5 kg/mm². This serves to improve the strength of theglass substrate and the durability against breakage based on aging. Thecompressive stress less than 1 kg/mm² undesirably weakens the strengthof the glass substrate (deterioration of the durability against defectsand the characteristics withstanding breakage) while the compressivestress over 15 kg/mm² enlarges the profile variation components andmakes it difficult to control the outer edge profile.

[0034] The tensile stress over 4.5 kg/mm² also enlarges the profilevariation components and make the control of the outer edge profiledifficult.

[0035] At any rate, the above-mentioned merits are more excellent bysetting the depth of the compressive stress layer, the compressivestress, and the tensile stress into optimum values. These are true ofthe inner peripheral side end.

[0036] According to a fifth aspect of this invention, the chemicalreinforcement condition defines a processing temperature and aprocessing time during the chemical reinforcement. By rendering theprocessing temperature and the processing time of the chemicalreinforcement condition into predetermined ranges, it is possible toreduce the profile variation components which appear on the outer and/orthe inner peripheral side ends during the chemical reinforcementprocessing.

[0037] In addition to the processing temperature and time, the chemicalreinforcement condition may be specified by a species of fused salts anda mixing ratio of the fused salts. However, the processing temperatureand time can be readily adjusted in comparison with the species and themixing ratio of the fused salts. Accordingly, controlling the processingtemperature and time is very effective on massproduction and inworkability.

[0038] According to a sixth aspect of this invention, it is preferablethat the processing temperature and the processing time fall with arange between 280° C. and 400° C. and a duration between 0.5 and 5hours, respectively. If the processing temperature is lower than 280°C., the processing temperature is undesirably lower than a melting pointof the fused salt or salts. On the other hand, the processingtemperature higher than 400° C. undesirably shortens the processing timeand gives rise to a reduction of workability. The processing timeshorter than 0.5 hour becomes worse in workability while the processingtime over 5 hours undesirably worsen productivity.

[0039] Preferably, the processing temperature and the processing timemay fall within ranges between 340 and 360° C. and between 1 and 4hours, respectively, so as to lower the profile variation components onthe outer and/or the inner peripheral side ends of the chemicallyreinforced glass substrate, although they can not be uniquely determinedbecause of depending upon glass compositions of the glass substrate,compositions of the chemical reinforcement solution, and so on.

[0040] According to a seventh aspect of this invention, the glasssubstrate prior to chemical reinforcement has a main surface chamferedand polished together with the edge portion adjacent to the outer and/orthe inner peripheral side end. In addition, the previously finding steppreviously finds or predicts, as the relationship, a relationshipbetween a polishing condition of the main surface and an edge profileobtained on the basis of the polishing condition. Moreover, the decidingstep obtains the glass substrate prior to chemical reinforcement bycontrolling the polishing condition of the main surface on the basis ofthe above-mentioned relationship between the polishing condition and theedge profile. Specifically, the glass substrate prior to chemicalreinforcement has the main surface chamfered along the outer and/or theinner peripheral side ends each adjacent to the edge portion. Predictionis made about the relationship between the polishing condition ofpolishing the main surface of the glass substrate subjected to achamfering process and the profile of the edge portion adjacent to theouter and/or the inner peripheral side end. It is posssible to obtainthe glass substrate prior to the chemical reinforcement, (may be calledan unreinforced or a provisional glass substrate), which has desiredouter and/or inner edge profiles by controlling the polishing conditionof the main surface on the basis of the above-mentioned relationship.

[0041] According to an eighth aspect of this invention, the polishingcondition is determined such that the edge portion is polished to be putinto a surface down state lowered relative to the main surface 2 of theglass substrate 1, as illustrated in FIG. 1. Such a polishing conditionof rendering the main surface 2 into the surface down state makes itpossible to simply and precisely obtain the provisional glass substratewhich has the outer edge profile removed by or cancelled by a profilevariation caused by the chemical reinforcement. This is true of theinner peripheral side end of the provisional glass substrate.

[0042] According to a ninth aspect of this invention, the polishingcondition determined for the surface down state is defined such that useis made about a soft polisher of a hardness between 60 and 80 (Asker-C)and a surface pressure to the glass substrate is kept at a range between40 and 150 kg/cm² during polishing. The above-mentioned polishingcondition makes it possible to readiyl and precisely attain the glasssubstrate prior to the chemical reinforcement, which stably keeps thesurface down state and to readily control the outer edge profile. Inaddition, it has been found out that the outer edge profile of the edgeportion tends to be rendered into the surface down state as the polisheror a polishing pad is hardened with the other conditions kept intact.The outer edge profile is liable to become the surface rise state as thepolishing pressure becomes high while the outer edge profile is renderedinto the surface down state as the polishing rotation speed becomeshigh.

[0043] The outer edge profile is varied in dependency upon polishingconditions determined by a structure, a size, and an amount of abrasivematerials of a polishing machine. However, controlling the outer edgeprofile by the hardness of the polisher has a good controllability andis readily executed over a wide range. Under the circumstances, it ispreferable that the outer edge profile may be mainly controlled by thehardness of the polisher and subordinately controlled by the polishingpressure and the polishing speed.

[0044] According to a tenth aspect of this invention, a glass substrateis subjected to chemical reinforcement and is for use in an informationrecording medium. The glass substrate chemically reinforced has a mainsurface and an edge portion which is adjacent to an outer and/or aninner peripheral side end and which is contiguous to the main surface,The edge portion of the glass substrate which is chemically reinforcedhas a predetermined region defined by a profile which falls within ±0.35μm in relation to a flat portion of the main surface determined as areference surface (zero). With this structure, it is possible to make amagnetic head stably float and run without any head crashes with a lowheight left and to widen a recording area. At any rate, high densityrecording and reproducing can be achieved. This means that the glasssubstrate after the chemical reinforcement is kept flat in the outeredge profile to the exent that no problem takes place in connection withthe extension of the recording area and the high density recording andreproducing. This is very effective in a magnetic recording medium of aLUL type.

[0045] In the meanwhile, the predetermined region adjacent to the outerperipheral side end may be optionally determined along the outerperipheral side end. However, it is preferable that the predeterminedregion may be defined by a region which is largely deviated from areference surface which is determined by a flat portion of the mainsurface and which is roughened in flatness.

[0046] Specifically, it is possible to define, as the predeterminedregion, a region between an outer periphery of a recording area (an areaof the main surface usually keeping flatness) on the main surface and anoutermost periphery of a glide region determined on the main surface.Alternatively, the predetermined region may be determined by a regionfrom a side end wall of the glass substrate to an inside of the outerperiphery of the recording area.

[0047] Herein, it is to be noted that the recording area is generallyincluded within the glide region but may be identical with the glideregion.

[0048] At any rate, It is preferable that the outer edge profile fallswithin a range of ±0.20 μm (namely, between −0.20 μm and +0.20 μm) andmore preferably, a range of ±0.10 μm (−0.10μm and +0.10μm). This alsoapplies to the inner peripheral side end.

[0049] According to an eleventh aspect of this invention, the glasssubstrate is subjected to chemical reinforcement and defined as a glasssubstrate after chemical reinforcement. The main surface of the glasssubstrate after chemical reinforcement has a glide area which includes arecording area located inside of the glide area. The glide area has aglide outer periphery while the recording area has a recording areaouter periphery inside the glide outer periphery and a flat area. Theedge profile chemically reinforced has, within an area extended from theglide outer periphery to an inside of the recording area, a ski-jumpedpoint which is the highest point with respect to a reference surface(zero) defined by the flat area and which has a ski-jump value notgreater than ±0.35 μm at the ski-jump point. In addition, the edgeprofile also has, at a roll-off point defined by a position of the glideouter periphery, a roll-off value not greater than ±0.35 μm with respectto the reference surface.

[0050] As mentioned above, the ski-jump point and the roll-off point aredefined in the area which is extended from the glide outer periphery toan inside of the recording area. This means that it is possible torender the ski-jump value and the roll-off value into less than +0.35 μmand more than −0.35 μm, respectively, with respect to the referencesurface, The edge profile which has the above-mentioned ski-jump valueand roll-off value can attain the merits mentioned with reference to thetenth aspect before. In addition, it is possible to readily manageproducts by executing numerical control in consideration of the ski-jumppoint and the roll-off point.

[0051] Specifically, the ski-jump value represents a value of theski-jump point which is the highest point on the edge profile, withrespect to the reference to the flat surface of the glass substratewhile the roll-off value represents a value of the roll-off pointdetermined on a border line drawn at a position of the glide outerperiphery, as mentioned before. The roll-off value is also decided withrespect to the reference surface.

[0052] Both the ski-jump value and the roll-off value are measured inthe following manner.

[0053] As shown in FIG. 2, consideration is made about a section of theglass substrate cut by a plane which is perpendicular to the mainsurface of the glass substrate and which passes through a center of theglass substrate of a disk shape. Within the section, two referencepoints are determined within the recording area of the main surface onan outline of the recording area and are successively named ?R1 and ?R2from the order near to the center of the glass substrate. In addition,an additional point ?R3 is determined on a line extended from therecording area outer periphery in an outer direction and is remote fromthe recording area outer periphery by a predetermined distance. Theadditional point R3 defines a position of the glide outer periphery ofthe glide area.

[0054] Next, the reference points R1 and R2 are connected to each otherby a line which is extended outwards of the glass substrate. Theextended line is drawn by a broken line in FIG. 2. Within an areabetween R2 and R3, measurement is made about a distance between the lineR1R2 (or the extended line) and each point set on the outline of theglass substrate. The ski-jump point (represented by S) on the outline ofthe glass substrate is defined by a highest point at which the distanceis the highest in a positive direction. The distance s at the ski-jumppoint is the ski-jump value.

[0055] On the other hand, the roll-off point is defined by a point Rwhich corresponds to R3 and which is placed on the outline of the glasssubstrate while a distance r between point R and the straight line R1R2(or the extened line) is representative of the roll-off value.

[0056] As shown in FIG. 3, it happens that the ski-jump value s slightlytakes a negative value. In this case, the ski-jump specifies the surfacedown state. As shown in FIG. 4, the roll-off value r also often takes apositive value, which specifies a surface rise state of the glasssubstrate. Moreover, the ski-jump value s may become equal to theroll-off value r, as shown in FIG. 4.

[0057] The reference points R1 and R2 and the point R3 may be optionallyselected with reference to a size of the glass substrate. For example,when the glass substrate has an outside diameter of 2.5 inches, 3.0inches, and 3.5 inches, the point R3 may be determined at a positionwhich is placed at 1 mm from the outer peripheral side end on an insideof the glass substrate. In the case of the glass substrate of 2.5 inches(65 mm in diameter), the reference points R1 and R2 and the roll-offpoint R3 may be determined, for example, at the positions of 23 mm, 27mm, and 32.5 mm from the center of the glass substrate, respectively.The roll-off point R3 in the above-mentioned example is determined onthe outer peripheral side end.

[0058] When the ski-jump value exceeds the range of ±0.35 μm, themagnetic disk can not accomplish stable floating and gives rise to headcrashes. This make it difficult to install the magnetic recording mediumwithin a magnetic disk drive.

[0059] The roll-off value over the range of ±0.35 μm also deterioratesfloating stability of the magnetic head and gives rise to head crashes.

[0060] Preferably, each of the ski-jump value and the roll-off valuefalls within a range of ±0.20 μm and more preferably, within a range of±0.10 μm.

[0061] According to a twelfth aspect of this invention, the glasssubstrate after chemical reinforcement has a compressive stress layerwithin a surface layer which is caused by the chemical reinforcement andwhich has a depth between 3 and 100 μm and a compressive stress of 1-15kg/mm²,

[0062] Moreover, the glass substrate after chemical reinforcement alsohas a tensile stress not greater than 4.5 kg/mm² within an inside of theglass substrate.

[0063] The depth of the compressive stress layer between 3 and 100 μmmakes it possible to manufacture an information recording medium glasssubstrate which has preferable mechanical strength. The depth less than3 μm weakens the mechanical strength of the glass substrate (durabilityagainst defects and characteristic withstanding breakage). When thedepth of the compressive stress layer exceeds 100 μm, the profilevariation component becomes large when the chemical reinforcement isperformed. Preferably, the depth of the falls within a range between 40and 80 μm and more preferably, within a range between 50 and 70 μm.

[0064] Second, the compressive stress caused in the surface layer of theglass substrate by the chemical reinforcement is selected between 1 and15 kg/mm² while the tensile stress caused within the glass substrate isnot greater than 4.5 kg/mm². This serves to improve the strength of theglass substrate and the durability against breakage based on aging. Thecompressive stress less than 1 kg/mm² undesirably weakens the strengthof the glass substrate (deterioration of the durability against defectsand the characteristics withstanding breakage) while the compressivestress over 15 kg/mm² enlarges the profile variation components andmakes it difficult to control the outer edge profile.

[0065] The tensile stress over 4.5 kg/mm² also enlarges the profilevariation components and make the control of the outer edge profiledifficult.

[0066] At any rate, the above-mentioned merits are more excellent bysetting the depth of the compressive stress layer, the compressivestress, and the tensile stress into optimum values. These are true ofthe inner peripheral side end.

[0067] According to a thirteenth aspect of this invention, a method ofmanufacturing an information recording medium from the glass substratecomprises the step of depositing a recording layer on the main surfaceof the glass substrate. The information recording medium thusmanufactured has the glass substrate flat on the outer edge profile anda wide recording area. In such an information recording medium, theglass substrate has a flat inner edge profile also and can avoidbreakage. In any event, the information recording medium can beappropriately mounted onto a magnetic memory device.

[0068] According to a fourteenth aspect of this invention, aninformation recording medium manufactured from the glass substratecomprises a magnetic layer over the main surface. The informationrecording medium has a high recording density because the glasssubstrate has a flat outer edge profile and can widen a recording area.The glass substrate which has the flat inner edge profile can avoidbreakage. In any event, the information recording medium can beappropriately mounted onto a magnetic memory device.

[0069] According to a fifteenth aspect of this invention, theinformation recording medium is available for a magnetic recordingmedium of a LUL drive type which can realize an extremely low floatingoperation of the magnetic head. Therefore, this invention is veryeffective when it is applied to the LUL drive type magnetic recordingmedium.

BRIEF DESCRIPTION OF THE DRAWING

[0070]FIG. 1 is a sectional view for use in describing a polished glasssubstrate which has an outer edge profile put in a surface down state;

[0071]FIG. 2 is a sectional view for use in describing an edge profilein detail;

[0072]FIG. 3 is a sectional view for use in exemplifying a ski-jump anda roll-off at an edge portion of a glass substrate;

[0073]FIG. 4 is a sectional view for use in exemplifying anotherski-jump and another roll-off at an edge portion of a glass substrate;

[0074]FIG. 5A is a sectional view of an edge profile of a glasssubstrate prior to a chemical treatment;

[0075]FIG. 5B is a sectional view of an edge profile of a chemicalreinforced glass substrate after the chemical treatment;

[0076]FIG. 6 is a flow chart of a method of manufacturing a chemicalreinforced glass substrate according to a preferred embodiment of thisinvention;

[0077]FIG. 7 is a table shows measured results for finding reinforcementrelationships with regard to 3.5 inch glass substrates;

[0078]FIG. 8 is a table shows measured results for finding reinforcementrelationships with regard to 2.5 inch glass substrates;

[0079]FIG. 9 is a table shows measured results for finding polishingrelationships; and

[0080]FIG. 10 is a graph shows a part of the measured results of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0081] Referring to FIGS. 5A and 5B, description will be at firstdirected to a glass substrate and a chemical reinforced glass substrate,respectively, for a better understanding of this invention.

[0082]FIG. 5A shows an outer edge portion of a glass substrate 10 priorto chemical reinforcement. The glass substrate 10 prior to chemicalreinforcement may be simply called a glass substrate. The illustratededge portion is assumed to have an ideal edge profile or shape. Theglass substrate 10 has a main surface 11, an outer peripheral surface12, and a chamfer 13 between the main surface 11 and the side surface12.

[0083] When the glass substrate 10 is immersed in a chemicalreinforcement solution (not shown), specific ions in the surfaces of theglass substrate 10 are replaced with other ions which are larger thanthe specific ions in ionic radii. As a result, each surface of the glasssubstrate 10 slightly expands as shown by arrows in FIG. 5A.Consequently, the glass substrate 10 is treated or processed into achemical reinforced glass substrate 10′ as illustrated in FIG. 5B.

[0084] As shown In FIG. 5B, it has been confirmed that a swell or a bump14 is formed between the main surface 11′ and the chamfer 13′. When thechemical reinforced glass substrate 10′ is used for a magnetic disk of ahard disk drive, a magnetic head of the hard disk drive flies over thebump 14. As a result, the flight of the magnetic head becomes unstable.Moreover, it is hard to fly the magnetic head at a low height becausethe magnetic head clashes with the magnetic disk. In addition, theski-jump portion 14 limits a flat area of the main surface 11′. The flatarea is used for a recording area of the magnetic disk.

[0085] Additionally, though another bump portion (not shown) is formedbetween the outer peripheral surface 12′ and the chamfer 13′, it isunrelated to the flight of the magnetic head and the recording area ofthe magnetic disk. Therefore, explanation of the bump portion betweenthe outer peripheral surface 12′ and the chamfer 13′ will be omittedhereinafter.

[0086] According to Applicants' experimental studies of the chemicalreinforcement for the glass substrate, it has been found out that thebump portion of the outer edge portion of the chemical reinforced glasssubstrate could be controlled by changing chemical reinforcementconditions. In addition, the applicants have found that changingpolishing conditions could control the outer edge profile of the glasssubstrate having a chamfer. Taking this into consideration, theapplicants have completed a method according to this invention relatedto the chemical reinforced glass substrate on the basis of theabove-mentioned facts.

[0087] Furthermore, the applicants have found out that the flight of themagnetic head became stable and could becomes lower without clash withthe magnetic disk including the chemical reinforced glass substrate whenthe bump portion is appropriately controlled within a desirable size.Then the applicants have completed another method according to thisinvention related to the magnetic disk having the chemical reinforcedglass substrate on the basis of the findings.

[0088] Referring to FIG. 6, the description will proceed to a method ofmanufacturing a chemical reinforced glass substrate according to apreferred embodiment of this invention.

[0089] At the step S201 of FIG. 6, a plurality of glass substrates areprovided to be polished under various polishing conditions. Each of theglass substrates has a disk shape and a center hole. Namely, each of theglass substrates has an outer edge portion and an inner edge portion.Hereinafter, though the description is mainly made about the outer edgeportion, the inner edge portion can be also processed in a similarmanner. Generally, the glass substrate has a main surface, an outerperipheral surface, and a chamfer between the main surface and outerperipheral surface. The main surfaces of the glass substrates arepolished under the various polishing conditions to be changed topolished glass substrates with polished edge profiles (i.e. smooth orundeformed edge profiles). Polishing relationships between the polishingconditions and the polished edge profiles are found prior to chemicalreinforcement.

[0090] At the step S202, the polished glass substrates (or otherpolished glass substrates) are provided to be chemically reinforcedunder various reinforcement conditions. The polished glass substratesare chemically reinforced under the various reinforcement conditions tobe chemically reinforced to obtain chemical reinforced glass substrateshaving reinforced edge profiles (i.e. deformed edge profiles). Treatmentrelationships between the treatment conditions and the treated edgeprofiles are found.

[0091] At the step S203, to obtain a chemical reinforced glass substratehaving a desirable edge profile, a combination of one of the polishededge profiles and one set of reinforcement conditions is decided orselected on the basis of the reinforcement relationships found at thestep S202.

[0092] At the step S204, another glass substrate is provided andpolished on the basis of the polishing relationships found at the stepS201. As a result, a polished glass substrate is obtained which has thedecided edge profile decided at the step S203.

[0093] At the step S205, the polished glass substrate is chemicallyreinforced under the condition of the set decided at the step S203.Ideally, as a result, the polished glass substrate is rendered into thechemical reinforced glass substrate having the desirable edge profile.

[0094] Thus, the chemical reinforced glass substrate having thedesirable edge profile is obtained. For mass production, the stepsS203-S205 are repeated or the steps 204 and 205 may be repeated once thedesirable edge profile is determined at the step 203.

[0095] Then, the chemical reinforced glass substrate is sent to a nextprocess. At the step S206, at least a recording layer is deposited onthe main surface of the chemical reinforced glass substrate to form amagnetic disk. The recording layer including a magnetic layer.

[0096] Finally, at the step S207, the magnetic disk is assembled into ahard disk drive.

[0097] According to the above-mentioned method, it is readily understoodthat deformed portions, namely, the bump 14 (FIG. 2) caused by thechemical reinforcement at the edge portions can be controlled bychanging the reinforcement conditions. This is because the reinforcementrelationships between the deformed portions (especially, in a directionof thickness) and the reinforcement conditions are previously found. Thedeformed portions have the edge profiles in the direction of thickness.

[0098] Herein, the chemical reinforcement is for strengthening orreinforcing the glass substrate. As already described before, thechemical reinforcement enhances mechanical strength and durability ofthe glass substrate. As the chemical reinforcement, an ion exchangemethod and a de-alkalization method are known. In the ion exchangemethod, specific ions included in surfaces of the glass substrate arereplaced with other ions in a chemical reinforcement solution. Becausethe other ions are larger than the specific ions in ionic radii, thesurfaces of the glass substrate expand. As a result, the edge portiontends to swell.

[0099] On the other hand, in the de-alkalization method, alkali ions areremoved from the surfaces of the glass substrate. Consequently, the edgeportion tends to become small. The deformation at the inner edge portionis almost equal to 80-90 percent of that at the outer edge portion.

[0100] Moreover, according to the method, the reinforced edge profilecan be strictly controlled by selecting the polished glass substrate.This is because the deformation caused by the chemical reinforcement canbe adjusted by polishing the glass substrate on the basis of thereinforcement relationships and polishing relationships.

[0101] In addition, according to the above mentioned method, a desirablepolished edge profile can be easily and precisely obtained by selectingthe polishing conditions. This is because the polishing relationshipsare previously found and detected.

[0102] The desirable polished edge profile is, for example, as shown inFIG. 3. That is, the outer edge portion has a surface down without asurface rise. Such an edge profile is suitable for offsettingdeformation caused by the chemical treatment. The edge profile shown inFIG. 3 is easily and precisely obtained by using soft polisher havinghardness of 60-80 (Asker-C) with a surface pressure 40-150 g/cm² at themain surface of the glass substrate. When the hardness of the polisherbecomes hard and other polishing conditions are fixed, the edge portionbecomes lower. Similarly, when a polishing rotation speed becomes fastwith other polishing conditions kept constant, the edge portion becomeslower. When the polishing pressure becomes large, the edge portionbecomes higher.

[0103] The hardness of the polisher can readily changed by exchangingthe polisher for another one. The hardness of the polisher can be finelychanged by the exchange. The control of the polishing by the exchange ofthe hardness of the polisher can be precisely performed in a wide range.

[0104] On the other hand, when the reinforcement conditions are selectedso that the deformation is smaller and the edge profile of the polishedglass substrate is flat, the deformation is considerably suppressed. Ifthe deformation is small, the edge profile is easy to control.Consequently, it is readily possible to manufacture large number of thechemical reinforced glass substrates having even quality.

[0105] When a compressive stress layer caused by the chemical treatmenthas a depth of 3-100 μm, the chemical reinforced glass substrate hasnecessary strength and the deformation at the outer edge portion issuppressed. When the compressive stress layer has the depth smaller than3 μm, the chemical reinforced glass substrate can not withstand frictionand is easily broken. When the compressive stress layer has the depthlarger than 100 μm, the deformation is large and hard to control. Adesirable depth of the compressive stress layer falls within a range of40 μm to 80 μm and a more desirable depth falls within a range of 50 μmto 70 μm.

[0106] Moreover, when the compressive stress layer has a compressivestress 1-15 kg/mm² and a tensile stress caused by the compressive stresslayer in the glass substrate except for the compressive stress layer isunder 4.5 kg/mm², the glass substrate is improved in the strength andthe durability.

[0107] When the compressive stress is lower than 1 kg/mm², the glasssubstrate cannot withstand friction and is easily broken. When thecompressive stress is larger than 15 kg/mm², the deformation is largeand hard to control the edge profile. When the tensile stress is largerthan 4.5 kg/mm², the deformation is large and hard to control the edgeprofile.

[0108] The depth of the compressive stress layer, the compressivestress, and the tensile stress may be adjusted to improve the strengthand the durability of the glass substrate and to simplify the control ofthe edge profile. For example, it is desirable that the depth of thecompressive stress layer is 40-80 μm, the compressive stress is 3-14kg/mm², and the tensile stress layer is under 2.5 kg/mm².

[0109] The reinforcement conditions include reinforcement or treatmenttemperature, reinforcement or treatment time, a combination of fusedsalts, a mixed ration of the fused salts, and so on. The treatmenttemperature and the treatment time are easy to change and effective inmass production of the glass substrate. Namely, it is easy to suppressthe deformation of the edge portion within a predetermined range bysuitably selecting the treatment temperature and the treatment time. Forexample, the treatment temperature is 280-400° C. and the treatment is0.5-5 hours. If the treatment temperature is lower than 280° C., it islower than the fusion points of the fused salts. If the treatmenttemperature is higher than 400° C., the treatment time must be shortenedand must be strictly controlled. On the other hand, when the treatmenttime is shorter than 0.5 hours, it must be strictly controlled and anoperation is hard to be performed. When the treatment time is longerthan 5 hours, productivity is decreased. The treatment temperature andthe treatment time must be changed according to components of the glasssubstrate and components of the chemical reinforcement treatmentsolution. Accordingly, the treatment temperature and the treatment timefor suppressing the deformation cannot be generalized. However, andesirable example is cited soon. It is desirable that the treatmenttemperature is 320-380° C. (more desirably, 340-360° C.) and thetreatment time is 1-4 hours.

[0110] As already described in conjunction with FIG. 4, the deformationon the edge portion can be specified by the edge profile that can berepresented by the ski-jump and the roll-off.

[0111] Referring FIGS. 7 through 10, description will be made aboutexamples of this invention, taking the above into account.

EXAMPLE 1

[0112] In a first example, the reinforcement relationships are measured.The measurement is performed about the chemical reinforcement conditions(i.e. the chemical reinforcement temperature and the chemicalreinforcement time), variations on the basis of the deformation (i.e.variations of the outside diameter and the inside diameter), a thickness(or depth) of the compression stress layer, the compressive stress, thetensile stress, and transverse rupture strength.

[0113] Glass substrates prior to chemical reinforcement (simply calledglass substrates) are prepared for the measurement. Samples 1-1 to 1-7of FIG. 7 have 3.5 inches (i.e. 95 mm) in diameter while samples 2-1 to2-8 of FIG. 8 have 2.5 inches (i.e. 65 mm) in diameter. These glasssubstrates are polished and substantially have flat main surfaces. Underthe circumstances, measurement is made about both of the ski-jump values and the roll-off value r of each glass substrate which are defined inconjunction with FIG. 4 and which are about zero.

[0114] The roll-off point R is decided (see FIG. 4) at a position placedinside by 1 mm from the outer peripheral surface while the referencepoint R2 is decided at a position remote by 5.5 mm from the outerperipheral surface in connection with each glass substrate. In each ofthe 3.5 inch sample glass substrates, the point R3 is located at aposition remote by 46.5 mm from the center and the reference point R2 islocated at a position remote by 42 mm from the center.

[0115] In each of the 2.5 inch sample glass substrates, the point R3 islocated at a position distant by 31.5 mm from the center and the pointR2 is located at a position remote by 27 mm from the center.

[0116] A height of the outer edge portion is successively measured alongthe outline of the outer edge portion by a surface roughness measuringapparatus (SURFTEST SV-624 manufactured by Mitutoyo Co.). The height ofthe outer edge portion of the chemical reinforced glass substrate isdifferent from the that of the glass substrate prior to chemicalreinforcement. Herein, the highest point and its value of the edgeprofile between the points R2 and R3 is defined as the ski-jump point Sand the ski-jump value s, respectively, as mentioned before, and theski-jump value s is determined at every one of the chemical reinforcedglass substrates.

[0117] The outside and the inside diameters are measured by amicrometer. to obtain a first outside diameter prior to chemicalreinforcement and a second outside diameter of the chemical reinforcedglass substrate. The first outside diameter is different from the seconddiameter. Thus, a variation between the first and the second outsidediameters is obtained. Similarly, measurement is made about the insidediameters of the glass substrates prior to chemical reinforcement andafter chemical reinforcement to obtain first and second insidediameters, respectively. As a result, the first inside diameter isdifferent from the second inside diameter. Thus, a variation between thefirst and the second inside diameters is obtained.

[0118] In the above-mentioned example, a mixture of potassium nitrate(60 wt %) and sodium nitrate (40 wt %) is used as the chemicalreinforcement solution.

[0119] Results of the measurement are shown in FIGS. 7 and 8.

[0120] As easily understood from FIGS. 7 and 8, as the reinforcementtemperature becomes high, each variation of the outside diameter, insidediameter, and the ski-jump value s becomes large. Similarly, as thereinforcement time becomes long, each variation of the outside diameter,inside diameter, and the ski-jump value s becomes large. The transverserupture strength becomes also large as the treatment temperature and/orthe treatment time becomes high and/or long.

[0121] Taking the above into consideration, it is possible to obtain thedesirable edge profile of the chemical reinforced glass substrate in thefollowing manner. Namely, the chemical reinforcement conditions are atfirst determined within a range that satisfies mechanical and chemicaldurability required for the glass substrate for the magnetic disk or themagnetic recording medium. Thereafter, a ski-jump value depending uponthe chemical reinforcement conditions is predicted in accordance with acorrelation between the determined chemical reinforcement conditionsmentioned above and a variation of the ski-jump values. Under thecircumstances, the edge profile of the glass substrate prior to chemicalreinforcement is determined as the desirable edge profile inconsideration of both the lapping process and the polishing process.Thus, the edge profile of the chemical reinforced glass substrate can bestrictly controlled.

[0122] Required transverse rupture strength of the glass substrate forthe magnetic disk of 3.5 inches falls within a range between 15 and 20kgf. All of the samples 1-1 through 1-7 have the transverse rupturestrength falling within the above-mentioned range. From this fact, it isconcluded that the variation of the outer edge profiles can be reducedor suppressed by performing the chemical reinforcement under thechemical reinforcement conditions that the reinforcement temperature andthe time fall within the range between 340 and 360° C. and the rangebetween 1.5 and 2 hours, respectively, and the variation of the ski-jumpvalues is small and falls within a range between 0 and 0.010 μm. Moredesirable conditions are specified by combinations of the reinforcementtemperature of 340° C. and the reinforcement time 1.5-2 hours.

[0123] On the other hand, the required transverse rupture strength ofthe glass substrate for the magnetic disk of 2.5 inches is about 10-15kgf. In this case, all of the samples 2-1 through 2-8 satisfy therequired transverse rupture strength. Then, the desirable conditionswhich can suppress the variation are combinations of the reinforcementtemperature of 340-360° C. and the reinforcement time 0.6-2 hours.

[0124] From the above-mentioned results, conclusion may be made aboutthe facts that the preferable chemical reinforcement conditions forsatisfying the required mechanical strength with the ski-jump value keptsmall are that the depth of the compressive stress layer caused by thechemical reinforcement is present between 40 and 80 μm, the compressivestress is between 3 and 14 kg/mm², and the tensile stress is not greaterthan 2.5 kg/mm². The tensile stress is caused by the compression stresslayer in the chemical reinforced glass substrate except for thecompression stress layer.

EXAMPLE 2

[0125] In a second example, the polishing relationships are measured. Aplurality of polishers having different hardness is prepared formeasuring the roll-off value r. For each of polishers, one hundred ofthe glass substrates unpolished are provided. Each unpolished glasssubstrate has a flat surface and 3.5 inches in diameter. In this event,an amount of each polisher and a polishing pressure are kept constant.In each unpolished glass substrate, the point R3 is decided at aposition remote by 1 mm from the outer peripheral side surface. Underthe circumstances, the roll-off value r at the point R3 is determined ormeasured which represents an amount of deviation of the outline point Rfrom the reference line or surface.

[0126] Results of the measurement are shown in FIGS. 9 and 10. As easilyunderstood from FIGS. 9 and 10, when the hardness is smaller than 60(Asker-C), the roll-off value r is positive. With an increase of thehardness, the roll-off value r becomes low.

[0127] As mentioned before, it is readily understood that the ski-jumpvalue s becomes positive by chemical reinforcement when the glasssubstrate is subject to the chemical reinforcement. Taking this intoaccount, it is necessary to put the edge profile of the glass substrateprior to the chemical reinforcement (namely, the glass substrate afterthe polishing process) into the surface down state in relation to themain surface of the above-mentioned glass substrate in order to keep theedge profile of the chemical reinforced glass substrate in a flat orexcellent state. Accordingly, the polisher must have the hardness over60 (Asker-C). Desirable hardness of the polisher is 60-80 (Asker-C).More desirable hardness of the polisher is between 66 and 80 (Asker-C)because the mean value of the roll-off values r is negative.

EXAMPLE 3

[0128] In the example 3, a chemical reinforced glass substrate ismanufactured and a magnetic disk is manufactured by the use of thechemical reinforced glass substrate.

[0129] (1) ROUGH LAPPING PROCESS

[0130] At first, melted glass (e.g. aluminosilicate glass) is pressed byan upper mold, a lower mold, and a body mold to form a glass substratehaving 96.0 mm in diameter and 1.8 mm in thickness. The aluminosilicateglass comprises, as main components, silicon dioxide (SiO₂:58-75 wt %),aluminum oxide (Al₂O₃:5-23 wt %), lithium oxide (Li₂O: 3-10 wt %), andsodium oxide (Na₂O:4-13 wt %). For example, the aluminosilicate glassincludes SiO₂ of 63.5 wt %, Als₂O₃ of 14.2 wt %, Li₂O of 5.4 wt % Na₂Oof 10.4 wt %, ZrO₂ (zirconium dioxide) of 6.0 wt %, Sb₂O₃ (diantimonytrioxide) of 0.4wt %, and As₂O₃ (diarsenic trioxide) of 0.1 wt %.

[0131] The glass substrate may be obtained by grinding or cutting asheet glass with a grinder. The sheet glass is formed by, for example, adown flow method or a float method.

[0132] Next, the glass substrate is lapped by a lapping machine. Namely,the glass substrate is mounted on a carrier of the lapping machine.Then, both sides (upper and lower main surfaces) are lapped by the useof abrasive grains having a particle size of #400. The abrasive grainsare, for example, aluminum oxide. In this process, the glass substrateis loaded with 100 kg. Thus, the substrate is changed into a roughlylapped glass substrate having profile irregularity of 0-1 μm and surfaceroughness of 6 μm in Rmax (based on JIS B 0601).

[0133] (2) SHAPING PROCESS

[0134] An opening is formed at the center of the roughly lapped glasssubstrate by a cylindrical whetstone and an inner peripheral surface isexposed. Moreover, an outer peripheral surface of the roughly lappedglass substrate is ground so that the outside diameter becomes 95 mm. Inaddition, chamfers are formed between each main surface and the innerperipheral surface and between each main surface and the outerperipheral surface. The inner peripheral surface, the outer peripheralsurface, and chamfers have surface roughness 4 μm in Rmax. Thus, theroughly lapped glass substrate is rendered into a shaped glasssubstrate.

[0135] (3) MIRROR FINISHING PROCESS FOR EDGES

[0136] While the shaped glass substrate is rotated, the inner peripheralsurface, the outer peripheral surface, and the chamfers are brushed withslurry of cerium dioxide abrasive grains. Thus, mirror finish for theinner peripheral surface, the outer side surface, and the chamfers iscompleted. The inner peripheral surface has surface roughness of 0.17 μmin Rmax and 0.02 L m in Ra. The chamfers of the inner edge portion havesurface roughness of 0.60 IL m in Rmax and 0.08 μm in Ra. The outerperipheral surface has the same surface roughness as the innerperipheral surface has. The chamfers of the outer edge portion havesurface roughness of 0.77 μm in Rmax and 0.10 μm in Ra. The surfaceroughness is measured by a measurement apparatus of Tencor P2(manufactured by KLA-Tenkor Co.)

[0137] Then the mirror finished glass substrate are washed with water.

[0138] (4) LAPPING PROCESS

[0139] The mirror finished glass substrate is lapped by the lappingmachine with abrasive grains having a particle size of #1000. By thisprocess, the mirror finished glass substrate changes into a smoothlapped glass substrate having flatness of 3 μm and surface roughness ofabout 2.0 μm in Rmax and 0.2 μm in Ra. The surface roughness is measuredby an AFM (interatomic force microscope).

[0140] Then, the smooth lapped glass substrate is soaked in neutraldetergent liquid and water in order and washed.

[0141] (5) POLISHING PROCESS

[0142] The smooth lapped glass substrate is polished by polishingmachine to remove flaws and warps that cannot be removed by theabove-mentioned lapping process. This process is performed on the basisof the results of the examples 1 and 2. For example, the ski-jump values increases by 0.004 μm when the chemical reinforcement treatment isperformed at 340° C. for 2 hours. In this case, if the polishing isperformed so that the ski-jump value s is about 0 μm and the roll-offvalue r is about −0.004 μm, the edge profile becomes almost flat.Namely, the polishing conditions that the ski-jump value s is about 0 μmand the roll-off value r is about −0.004 μm are selected. Additionally,the points 2 and 3 correspond to those of Examples 1 and 2.

[0143] The selected polishing conditions are enumerated below.

[0144] Polishing Fluid: cerium dioxide (mean particle diameter=1.0 μm)(free abrasive grains+water),

[0145] Polisher: Soft Polisher (hardness of 68 (Asker-C)),

[0146] Load: 200 kg (surface pressure: 66 g/cm²),

[0147] Polishing Time: 80 minutes,

[0148] Removing thickness: 50 μm,

[0149] Rotation Speed of Upper Table: 20 rpm,

[0150] Rotation Speed of Lower Table: 26 rpm,

[0151] Revolution (orbital motion) Speed of Carrier: 3 rpm, and

[0152] Rotation Speed of Carrier: 3 rpm.

[0153] After the polishing under the above mentioned conditions isperformed, the polished glass substrate is successively soaked inneutral detergent liquid, first pure water (demineralized water), secondpure water, first isopropyl alcohol, and second isopropyl alcohol (steamand drying) in order.

[0154] The ski-jump value s and the roll-off value r of the polishedglass substrate are measured by the surface roughness measuringapparatus (SURFTEST SV-624 manufactured by Mitutoyo Co.). The ski-jumpvalue s is equal to +0.002 μm while the roll-off value r is equal to−0.005 μm. It can be recognized that the edge portion practically has asurface down without a surface rise.

[0155] (6) CHEMICAL REINFORCEMENT TREATMENT PROCESS

[0156] The polished glass substrate is heated to 300° C. and soaked inchemical reinforcement solution heated to 340° C. for 2 hours. Thechemical reinforcement solution is a mixture of potassium nitrate (60%)and sodium nitrate (40%).

[0157] Next, the chemical reinforced glass substrate is soaked in waterof 20° C. for 10 minutes to cool it quickly. In this event, the chemicalreinforced glass substrate is broken if it has minute cracks on thesurfaces. That is, this process enables elimination of a defectivearticle.

[0158] Then, the chemical reinforced glass substrate is soaked insulfuric acid having concentration of 10 wt %, neutral detergent liquid,first pure water, second pure water, and isopropyl alcohol in order towash it.

[0159] The ski-jump value s and the roll-off value r of the chemicalreinforced glass substrate are measured. The ski-jump value s and theroll-off value r are equal to +0.002 μm and +0.005, respectively. Thismeans that the surface of the chemical reinforced glass surface betweenthe points R2 and R3 falls within a range of −0.005 μm and +0.005 μmfrom the reference surface. That is, the chemical reinforced glasssurface has a nearly flat surface at the edge portion.

[0160] Moreover, the chemical reinforced glass substrate has thecompression stress layer of 79.8 μm thick, the compressive stress of13.8 kg/mm², and the tensile stress of 2.0 kg/mm². The values are equalto those of the sample 1-2 of FIG. 7.

[0161] Furthermore, the chemical reinforced glass substrate has thesurface roughness Ra of 0.51 nm and Rmax of 5.20 nm on the main surface,waviness Wa of 0.43 nm at the main surface, and mircowaviness Wa (Ra) of0.50 measured at a microscopic area of the edge portion. Additionally,the surface roughness Ra and Rmax are measured by the interatomic forcemicroscope. The waviness Wa is measured by a multifunction diskinterferometer (e.g. OPTIFLAT manufactured by PHASE SHIFT TECHNOLOGYCo.). The measurement of the waviness Wa is performed about an area(about 115400 pixels) of 20.3 mm to 45.0 mm remote from the center in aradius direction. The microwaviness Wa (Ra) is measured by amultifunction surface analyzing machine (e.g. MicroXAM manufactured byPHASE SHIFT TECHNOLOGY Co.). The measurement of the waviness Wa (Ra) isperformed to a square area (about 250000 pixels) of about 500 μm inlength and about 600 μm in width.

[0162] The vaviness Wa and the microwaviness Wa (Ra) are provided asfollows.

[0163] The microwaviness Wa (Ra) and the waviness Wa are calculated byscanning a predetermined area of the substrate surface by the use ofwhite light (wavelength: about 550 nm) and etc., by synthesizingreflection light from the substrate surface with reflection light from areference surface, and by using interference fringes appearing at asynthesized point.

[0164] The microwaviness Wa (Ra) on the micro-area has a waviness periodof about 2 μm to 4 mm and is represented by an average of an absolutevalue of a deviation from a center line to a measured curve. Herein, thecenter line is defined by a straight line such that, when a parallelstraight line is drawn which is parallel to an average line of themeasured curve, an area surrounded by the parallel straight line and themeasured curve becomes equal to each other on both sides of the parallelstraight line. The microwaviness Wa (Ra) and the waviness Wa may bemeasured by any other light than white light.

[0165] A plurality of measuring points is decided on the measuringcurve. Each of the measuring points has a measuring point value. Themeasuring point value is a distance from a standard straight line whichis suitably decided. The microwaviness Wa (Ra) (or waviness Wa) is givenby:${{{Wa}({Ra})}\left( {{or}\quad {Wa}} \right)} = \left. {\frac{1}{N}\sum\limits_{i = 1}^{N}}\quad \middle| {{Xi} - \overset{\_}{X}} \right|$

[0166] where N is the number of the measuring points, Xi is themeasuring point value at the i-th measuring point, and {overscore (X)}is the mean of all of the measuring point values X.

[0167] It is preferable that the microwaviness Wa (Ra) and the wavinessWa have the microwaviness period from 2 μm to 4 mm, as mentioned before,and a waviness period from 300 μm to 5 mm, respectively.

[0168] (7) MANUFACTURING PROCESS FOR MAGNETIC DISK

[0169] A nickel-aluminum seed layer, a chrome-molybdenum underlayer, acobalt-chrome-platinum-tantalum magnetic layer, and carbon hydrideprotective layer are deposited in order on the both sides (upper andlower main surfaces) of the chemical reinforced glass substrate by asputtering apparatus of an inline type. A perfluoropolyether lubricantlayer is formed each carbon hydride protective layer by the use of a dipmethod. Thus, a magnetic disk is completed.

[0170] The magnetic disk is mounted on a hard disk drive of the LULmethod. In the hard disk drive, a magnetic head can fly stably withoutclash with the magnetic disk. Because the surface roughness Ra and Rmaxand the waviness Wa and the microwaviness Wa (Ra) are all small, atouch-down-height is preferably not greater than 10 nm. Thus, therecording area can be expanded. ?

COMPARATIVE EXAMPLE 1

[0171] A chemical reinforced glass substrate is manufactured by a mannersimilar to the example 3. However, the polishing process is performed sothat the roll-off value r becomes positive by adjusting especially theouter edge profile. In addition, the chemical treatment process isperformed at 380° C. for 4 hours.

[0172] Though the chemical reinforced glass substrate has the surfaceroughness Ra and Rmax that are similar to those of the example 3, theski-jump value s and the roll-off value r are equal to +0.421 μm and+0.420 μm, respectively. When the chemical reinforced glass substrate isused for a magnetic disk of a hard disk drive, a magnetic head clasheswith the magnetic disk.

[0173] The chemical reinforced glass substrate has a compression stresslayer of 140.8 μm in depth, a compressive stress of 20.5 kg/mm² incompression stress layer, a tensile stress 2.7 kg/mm² in the chemicalreinforced glass substrate. These values are equal to those of thesamples 1-7 of FIG. 7. Moreover, the chemical reinforced glass substratehas the waviness Wa of 1.85 nm and the microwaviness Wa (Ra) of 0.93 nm.

[0174] As understood from comparison of Example 3 with the comparativeexample 1, it is to be noted that the chemical reinforcement conditionsare decided in Example 3 in relation the required mechanical strengthand durability and the polishing conditions are decided so as to cancelthe deformation caused by the chemical reinforcement. In addition, thechemical reinforced glass substrate is manufactured in Example 3 on thebasis of the decided polishing condition and the decided chemicalreinforcement conditions. As a result, it is proved that the magnetichead does not clash with the magnetic disk that uses the chemicalreinforced glass substrate (Example 3) and that is driven by the harddisk drive adopting the LUL method. However, when the polishingconditions are not considered like in the comparative example, thechemical reinforced glass substrate can not be used for the magneticdisk driven by the LUL method.

[0175] Moreover, it is understood from the comparison of the example 3with the comparative example 1 that the ski-jump value s and thewaviness Wa and the microwaviness Wa (Ra) become large when thedeformation becomes large (the reinforcement time becomes long and/orreinforcement temperature becomes high).

EXAMPLE 4

[0176] A chemical reinforced glass substrate is manufactured by a mannersimilar to the example 3. However, the polished glass substrate issoaked in hydosilicofluoric acid after the polishing process. Thechemical reinforced glass substrate is used for a magnetic disk of ahard disk drive adopting the CSS (contact-start-stop) method.

[0177] The chemical reinforced glass substrate has the ski-jump value sand the roll-off value r that are the same as those of the Example 3.The surface roughnesses Rmax and Ra of the chemical reinforced glasssubstrate are influenced by the hydosilicofluoric acid and equal to 7.8nm and 0.83 nm, respectively.

[0178] In the hard disk drive, a magnetic head can fly stably withoutclash with the magnetic disk using the chemical reinforced glasssubstrate. Moreover, it is possible to widen the recording area by usingthe chemical reinforced glass substrate according to Example 4.

EXAMPLE 5

[0179] A chemical reinforced glass substrate is manufactured by a mannersimilar to the example 3. Herein, it is to be noted that the polishingconditions are selected so that the ski-jump value s and the roll-offvalue r become about 0 μm and about −0.035 μm, respectively, byexpecting or considering the ski-jump value s and the roll-off value rwhich are obtained during the chemical reinforcement performed under thechemical reinforcement conditions (380° C. and 4 hours) mentioned inconjunction with Example 1. The chemical reinforcement is performedunder the chemical reinforcement conditions (380° C. and 4 hours) afterthe polishing process is performed under the above-mentioned polishingconditions.

[0180] Practically, the polishing process is performed by the use of apolisher having hardness of 80 (Asker-C). The load, the polishing time,and other conditions are appropriately adjusted.

[0181] The chemical reinforced glass substrate has the ski-jump value sof +0.03 μm and the roll-off value r of −0.04 μm both of which are worsethan those of the example 3. This is because the deformation caused bythe chemical reinforcement is too large to be controlled by thepolishing.

[0182] Though the ski-jump value s and the roll-off value r are worsethan those of the example 3, a magnetic head can fly stably withoutclash with a surface of a magnetic disk using the chemical reinforcedglass substrate in a LUL type hard disk drive. In addition, therecording area can be widened in Example 5 also.

[0183] Plural chemical reinforced glass substrates were manufactured bythe processes of the example 3 and the example 5. It was confirmed thatthe chemical reinforced glass substrates manufactured by the example 3had a small variation in the edge profile in comparison with thechemical reinforcement glass substrates according to the example 5 whenmeasurement was made about the edge profile. Consequently, the chemicalreinforcement glass substrates according to the example 3 had the edgeprofiles which were uniform and excellent as compared with the example5.

[0184] While this invention has thus for been described in conjunctionwith the preferred embodiment thereof, it will readily be possible forthose skilled in the art to put this invention into practice in variousother manners.

[0185] For example, this invention may be applied to not only the outeredge portion but also the inner edge portion.

[0186] Moreover, the treatment conditions may include not only thetreatment temperature and the treatment time but also the kind of thechemical reinforcement treatment solution. For example, the mixtureratio of the potassium nitrate and sodium nitrate may be changed.Moreover, the potassium nitrate or the sodium nitrate may be used as thechemical reinforcement treatment solution. Furthermore, sodium sulfate(Na₂SO₄), potassium sulfate (K₂SO₄), sodium bromide (NaBr), potassiumbromide (KBr), KNO₂, and NaNO₂ may be selectively used as the chemicalreinforcement solution.

[0187] Furthermore, the chemical reinforcement may be a de-alkalizationprocess. In this case, it is desirable that the polishing is performedso that the ski-jump value s and the roll-off value r become positive.

[0188] Still furthermore, other polishing may be performed to the mainsurface of the chemical reinforced glass substrate. In this case, thetreatment conditions are adjusted so that the desirable edge profile isobtained after the other polishing.

[0189] In addition, the chemical reinforced glass substrate may be usedfor an optical disk, a magneto-optical disk, and the like.

What is claimed is:
 1. A method of manufacturing a glass substrate foran information recording medium, the glass substrate having an edgeportion adjacent to an outer and/or an inner peripheral side end,comprising the steps of: previously finding a relationship betweenchemical reinforcement conditions and that profile variation at the edgeportion of the glass substrate which results from chemicalreinforcement; and performing the chemical reinforcement of the glasssubstrate on the basis of the relationship to obtain a chemicallyreinforced glass substrate.
 2. A method of manufacturing a glasssubstrate for an information recording medium, the glass substratehaving an edge portion adjacent to an outer and/or an inner peripheralside end, comprising the steps of: previously finding a relationshipbetween chemical reinforcement conditions and that profile variation atthe edge portion of the glass substrate which results from chemicalreinforcement; deciding a profile on the edge portion by predicting theprofile variation of the edge portion to obtain, as a glass substrateprior to chemical reinforcement, an glass substrate prior to chemicalreinforcement which has a decided profile and which is not subjected tothe chemical reinforcement; and performing the chemical reinforcement ofthe glass substrate prior to chemical reinforcement to obtain the glasssubstrate which has a desired profile at the edge portion.
 3. A methodas claimed in claim 1, wherein the chemical reinforcement performingstep is performed on conditions such that the profile variation on theedge portion becomes small.
 4. A method as claimed in claim 1, whereinthe chemical reinforcement performing step is performed under thechemical reinforcement condition such that a compressive stress layerformed on a surface layer of the glass substrate by the chemicalreinforcement reaches to a depth between 3 and 100 μm and has acompressive stress of 1-15 kg/mm² and that a tensile stress caused bythe chemical reinforcement within the glass substrate is not larger than4.5 kg/mm².
 5. A method as claimed in claim 1, wherein the chemicalreinforcement condition defines a processing temperature and aprocessing time during the chemical reinforcement.
 6. A method asclaimed in claim 5, wherein the processing temperature and theprocessing time fall with a range between 280° C. and 400° C. and aduration between 0.5 and 5 hours, respectively.
 7. A method as claimedin claim 2, the glass substrate prior to chemical reinforcement having amain surface chamfered and polished together with the edge portionadjacent to the outer and/or the inner peripheral side end, wherein thepreviously finding step previously finds, as the relationship, arelationship between a polishing condition of the main surface and anedge profile obtained on the basis of the polishing condition; wherein:the deciding step obtains the glass substrate prior to chemicalreinforcement by controlling the polishing condition of the main surfaceon the basis of the above-mentioned relationship between the polishingcondition and the edge profile.
 8. A method as claimed in claim 7,wherein the polishing condition is determined such that the edge portionis polished to be put into a surface down state lowered relative to themain surface of the glass substrate.
 9. A method as claimed in claim 8,wherein the polishing condition determined for the surface down state isdefined such that use is made about a soft polisher of a hardnessbetween 60 and 80 (Asker-C) and a surface pressure to the glasssubstrate is kept at a range between 40 and 150 kg/cm² during polishing.10. A glass substrate which is subjected to chemical reinforcement andwhich is for use in an information recording medium, the glass substratechemically reinforced having a main surface and an edge portion adjacentto an outer and/or an inner peripheral side end and contiguous to themain surface, the edge portion of the glass substrate which ischemically reinforced having a predetermined region defined by an edgeprofile which falls within ±0.35 μm in relation to a flat portion of themain surface determined as a reference surface (zero).
 11. A glasssubstrate as claimed in claim 10, the glass substrate being subjected tochemical reinforcement and defined as a glass substrate after chemicalreinforcement, the main surface of the glass substrate after chemicalreinforcement having a glide area which includes a recording arealocated inside of the glide area, the glide area having a glide outerperiphery while the recording area has a recording area outer peripheryinside the glide outer periphery and a flat area, wherein: the edgeprofile chemically reinforced has, within an area extended from theglide outer periphery to an inside of the recording area, a ski-jumpedpoint which is the highest point with respect to a reference surface(zero) defined by the flat area and which has a ski-jump value notgreater than ±0.35 μm at the skl-jump point; the edge profile alsohaving, at a roll-off point defined by a position of the glide outerperiphery, a roll-off value not greater than ±0.35 μm with respect tothe reference surface.
 12. A glass substrate as claimed in claim 10,wherein the glass substrate chemically reinforced has a compressivestress layer within a surface layer which is caused by the chemicalreinforcement and which has a depth between 3 and 100 μm and acompressive stress of 1-15 kg/mm²; the glass substrate chemicallyreinforced also having a tensile stress not greater than 4.5 kg/mm²within an inside of the glass substrate.
 13. A method of manufacturingan information recording medium from the glass substrate claimed inclaim 1, further comprising the step of: depositing a recording layer onthe main surface of the glass substrate.
 14. An information recordingmedium manufactured from the glass substrate claimed in claim 10,further comprising a magnetic layer over the main surface.
 15. Aninformation recording medium as claimed in claim 14, the informationrecording medium being a magnetic recording medium of a LUL drive type.